In Nick Malik’s follow up post The value of intermediation, part 2, he describes “composable services” as follows:

I would suggest that a service, when composable, (a) provides information in a manner that can be readily reused without requiring multiple calls to other services for interpretation, and/or (b) provides functionality that can be executed to produce specific semantics in an encapsulated manner without requiring references to multiple other services and without unknown or undesirable side effects.

I woud state that a service that does not comply with the above “suggestions” is not a service in the SOA sense. I don’t care much for the “composable” or “reusable” qualifiers that are being placed on the term “service”. I believe that it creates fractured definitions that don’t improve understanding.

Nick continues with assertions about the connection between SOA and Canonical Data Models:

I argue, passionately, that a service that does not leverage a Canonical data model (explicit or implicit) is not useful outside a small handful of very specific situations. Such a limited service provides some small benefits, but probably not more than a COM+ component would, and certainly not enough to justify all this interest in SOA. We get no business agility from this kind of service. Therefore, as an Enterprise Architect, you can create it, but I will not use it, nor will I look kindly on another application that does. Poor integration is just barely a half step up from no integration. Some would argue it is a step down.

Just to be clear, a Canonical Data Model is something that exists without dependence on any service. That is why I don’t agree with its use in SOA. Each service has its own message schema which makes use of its own data schema if you will. A data schema is the definition of structures that are used in more than one message type. The service controlls both of these schemas and decides when and how to version them, which versions to support on which endpoints, etc. It’s all part of the service’s autonomy. I’m not quite sure what an implicit canonical data model is.

I’m also unclear as to the “benefit” a service provides. Services are the way we model our business domain – they are therefore a part of our business. In my previous post on intermediation and SOA I gave an example of a Purchasing Service. This service does not really exist to provide something external to it with a “benefit”, it’s an inherent part of the structure of the business, much like the Purchasing Department of the company is. It participates in business processes but is not ruled by them. I really don’t see how you could compare that to any kind of technological component. In that respect, you don’t “use” a service. Neither do you really “integrate” them. Each service decides which processes it needs to be a part of, simply by choosing which events (of the EDA kind) it subscribes to. This leads to a kind of “distributed integration” that maintains service autonomy and has no single point of versioning or failure.

One of the comments quoted includes the statement “Intermediation greatly complicates any message exchange pattern other than request-response and pub-sub”. Like I described in my previous post, intermediation, if it even does occur, occurs behind service boundaries. Between services, you have the basic message exchange patterns of duplex request/response (one way messaging each time, with correlation between them) and pub/sub. You really don’t need anything else, and yesterday’s middleware already handled all of that for us. Most of the advanced ESB functionality isn’t needed between services.

The next example given discusses a banking scenario (well, more of an HR one really), which brings Nick to the following conclusion:

This process is neither pub-sub nor request-response. It is a long-running orchestration with compensating events. The process is NOT complicated by the ability to intermediate.

In fact, I would argue that nearly all valuable long running transactions MUST have the ability to intermediate in order to allow them to be composed, and recomposed, and orchestrated.

Long running anything, is a series of message exchanges that are tied together in some way. This could be as simple as having all the messages contain the same process ID in their body. “Compensating events” are just the result of business logic being run in a service, possibly changing its own state (updating those backend systems and applications), and sending out other messages. If there’s any intermediation of the kind Nick describes, it occurs within a service between its backend systems and applications. He also implicitly states that there is value in composing/orchestrating these “long running transactions”, apparently without having the service be involved. I don’t see it. Not the way I do my services. Once again, each service is responsible for itself – its part of the global “long running transaction”, ie business process. If one service were to choose to implement its cross-application workflows in a hard-coded manner, that should have no impact on any other services, nor should they be aware of it. That service may have been implemented poorly and as a result have difficulty responding to changing business conditions quickly, but that’s its own business. That implementation could easily be upgraded in the future, without changing the overall Service-Oriented Architecture.

Nick’s conclusion follows:

In conclusion, I don’t say that intermediation is a requirement of a service oriented architecture. But I do say that intermediation is a requirement of a service oriented architecture designed to deliver composability, and therefore, business agility.

Mine is a little different. When your SOA is made up of autonomous business-level services, you will have business agility. The implementations of some services may benefit through the use of intermediation, but it is not a top level concern.

Nick Malik has an interesting post on The value of intermediation in SOA where he starts out suggesting a couple of books that stand at the basis of much of today’s SOA thinking. I agree that far too few people seem to have read them.

In his previous post Is it service-oriented if the message cannot be intermediated, Nick defines intermediability as “SOA should give us the ability to intercept a message going from point A to point B, and react to that message without informing either end of that pipe.”. I’ll respond to this in due course.

Anyway, he continues on by saying “SOA [is] an architecture for Enterprise Application Integration.”

I can’t agree with that statement. The main reason is that EAI puts the application in the center, and that integrating existing applications one of the primary purposes of it. It is my assertion that in order to solve many of the problems that we are having today, we need to take a broader, business based view of the enterprise and model that with services. A service may be implemented with one or more applications. However, my experience has been that these services tend to use parts of existing applications, with multiple services using different parts of the same application. The reason for this is that the applications we have today, especially the ERP monoliths, do a lot, and at the same time, not everything. This is part of the reality that EAI tried to solve, but then got mired down in cross system hell. You just can’t solve poor business decomposition in the technology domain.

The value of putting services at the fore makes it possible to gradually phase out and evolve legacy applications, and migrate costly mainframe apps bit by bit without having these changes ripple out and break other services. The same is true for those systems’ data – backup strategies are defined at the service level, impacted primarily by their Service-Level Agreements.

While I whole-heartedly agree with what Nick has to say in terms of OO intermediation of the Dependency Injection variety, and that scaling up those same concepts in terms of messaging is the right way to go, I take issue with orchestration in the intermediation area. These “tactical changes” need to be done in the context of the top, business-level service strategy. That means that all logic belongs within a service. The “network” between services is just that, a “dumb” network – no business logic of any kind, just technological capabilities like knowing which physical server to route messages to.

In this spirit, I’d like to suggest an alternative solution to the example Nick gives. Here’s the scenario:

Let’s say that system 1 generates an invoice. It sends an event to the world saying “invoice here” and system 2 captures that message. System 2 asks for details about the invoice… perhaps it will place the information on a web site for internal support teams.

Let’s say that we are moving to a CRM solution in our internal support groups. We want to create the information in the CRM system related to the invoices that specific customers have been issued. We need to integrate these two systems. The existing web app needs to have a link to the CRM system’s data, to allow the user to move across easily.

And here is the solution he prescribes:

We can intercept the request for further information from the web app to the publisher. When the publisher responds with information about the invoice, we can insert the invoice in the CRM system, add a link to the CRM record for that invoice to the data structure, and resume our response to the web app. Assuming that our canonical schema has a field for ‘foreign key’, we have just integrated our CRM and web information portal… without changing either one.

Without getting into the business-level analysis of what the correct service decomposition might be, here’s what I suggest (although all of these “systems” might just end up within the same service, or having parts of them being used by multiple services).

First of all, have all information about the invoice available via the message only. This could be done by actually putting all the invoice data in the message, or by placing a URI instead where other systems can HTTP GET it from – REST style. This decreases coupling between the publisher and its subscribers. However, we haven’t solved the problem of our web apps getting access to the relevant data in the CRM system.

The solution presents itself at the business level. The invoice is not “complete” without the appropriate CRM data. Therefore, it does not make sense for a service to publish it that way. Let’s call this service the Purchasing Service. It would handle the workflow of receiving the first system’s event, adding the invoice to the CRM system, and taking the resulting full invoice data and publishing that. All external systems like the web apps would see just the final event. Orchestration, if there even is such a thing, occurs within the service boundary. This technological level intermedation isn’t even a blip at the business level. We can also imagine other services, say a Sales Service, that would use the CRM system as well.

In summary, when moving to SOA, intermediation provides many technological benefits in getting data and behavior to work across existing systems and applications, however it’s laregly a NO-OP at the service level. After phasing out many of those existing applications behind the service boundaries, the same service-level interactions would persist. Your Service-Oriented Architecture would not be any different. That’s the technical agility aspect of SOA.

I’ve looked over the example Guy put up in his blog on how to interact with external services from a workflow, and tried it out using asynchronous APIs with WCF. This is done quite simply by having our service accept an object as a parameter that it can call methods on (which has availability issues, but whatever). The object that we pass in is quite simply a reference to our own service. Anyway, I haven’t been able to get the SendActivity in WF to work with it. Bummer. I guess I’m “stuck” doing things the “old fashioned” way.

By having messages dealing with workflow contain the ID of the specific instance they refer to, we can do simple message to workflow mapping. When a message handler receives a message containing a workflow ID, it just goes to the workflow store and retrieves the object by its ID. Finally, it calls the Handle method on that object, and I’m done. Workflow classes are just state machines whose triggers are the arrival of a message.

Here’s some example code so you can see how simple it really is:

public class WorkflowMessageHandler : IMessageHandler<Stage1Msg>
{
	public void Handle(Stage1Msg msg)
	{
		using (IDBScope scope = this.DbServices.GetScope(TransactionOption.On))
		{
			IWorkflow wf = this.DbServices.Get(msg.WorkFlowID);
			wf.Handle(msg);

			scope.Complete();
		}
	}
}

You can see how easy it would be to take this and make it generic. Just define an IWorkflowMessage that inherits from IMessage and has a single property – WorkFlowID. Then we could have a BaseWorkflowMessageHandler<T> which would inherit from IMessageHandler<T> where T : IWorkflowMessage.

After that, it would be enough to have a class inherit from the base for a specific message and you’d be done, just like this:

public class Stage1Message : IWorkflowMessage { // WorkFlowID and other data };

public class WorkflowMessageHandler : BaseWorkflowMessageHandler<Stage1Msg> {}

I could even automate the creation of these message handlers given the set of messages that correspond to a workflow. I could then create all sorts of sexy designers on top of that.

But, seeing as there’s so little extra code to get long-running workflows to work with “asynchronous services”, I don’t think I’ll bother. I mean, why do I even need a “SendActivity”? It’s just a simple little call:

this.Bus.Send(msg);

It appears that clean designs don’t leave much to be draggy-dropped. Oh well. Your mileage may vary.

The issue of efficiently utilizing multi-core CPUs is gaining wider and wider mindshare. The debate on whether to use threads or processes is heating up. Savas is coming at it from the parallelism angle and I think that I’m with him on this.

When I base my designs around the Bus API, all my logic is both thread and process neutral. The implementation of the bus could be such that it uses thread safe queues for single process deployment. Named Pipes or even MSMQ could be used for multi-process communication. It’s all just an implementation detail.

It never ceases to amaze me the way these basic design principles show up fractally, at multiple levels of abstraction.

While going through the JavaSpace presentation I found on Owen Taylor’s blog, I kept saying to myself, “well, I can do that without a space”, until I got to one part of it.

The ability to introduce a new task at runtime without restarting any servers, and have new clients be able to send those tasks, and existing servers perform them. I never did that before.

This is very important when you’ve already got a system running and you want to expand on it. I think that this covers at least half of all the software work being done on the planet.

The interesting thing is that this ability comes in two parts – one design, the other technology.

In terms of design, in order for existing servers to be able to do the work of the new task without any kind of restart, we need the code for performing the work of the task to reside in the task itself, possibly in some kind of “execute” method. The server would simply take a task out of the “queue” of pending tasks and call that execute method.

Now, those of us trying to do this with Microsoft technology know that if the assembly containing the code for that task was not available on that machine, this wouldn’t work. Technologically speaking, we’d receive some kind of deserialization exception that resulted from a TypeNotFoundException. In other words, in order to support the new task, we’d need to “install” all of its participating assemblies on all our servers.

For those of us who know and use Jini with Java, we get this behavior automatically – the bytecode of the task is downloaded automatically. This is an advantage in terms of operations, but I’m not an operations guy so I can’t say how huge this really is.

The interesting thing for me in this design is that it’s somewhat different from the messaging paradigm I’ve been so successful with. Here we don’t use messages as simple Data Transfer Objects. Tasks contain both data and behavior. This behavior used to belong to message handlers. I like having separate message handlers, it enables me to create very flexible pipelines.

Here’s how I’d trade it off. When using Jini, the tasks style gives me zero footprint deployment. When using .NET, if I’m already installing things on my existing servers, I can deploy new messages and handlers just as well as the task assemblies. The problem is that I’d need some way for the server to pick up on the new handlers – a background thread scanning the deployment directory, or the FileSystemWatcher (given the fact that it sometimes misses things).

Well, it looks like both design styles are feasible on both platforms. The ability to have code download automatically on the Java platform is a plus that is most felt when using tasks.

Bottom line so far, there’s a lot to learn from JavaSpaces, and even if you don’t use Java, Jini, or Space technologies (like this), the design patterns employed there are extremely valuable.

So many gems in Steve Jones’ recent post SOA isn’t about technology.

business process isn’t everything.

one interface = one process = what a load of crap interfaces you have.

… with “services exposing legacy and BPM orchestrating services”. Its pretty amazing how this view just happens to match the product vendors stacks…

Its bollocks

BPM as the language of business is, IMO, snakeoil.

Starting with BPM is as silly as starting with WSDL.

I’ve talked about the BPM, SOA divide in this podcast, but from a more solutions architecture perspective. As well as this podcast on “Services as the interface to business processes”.

The main problem I have with BPM is that there’s this expectation that the steps of each process will actually be implemented the way that they were drawn. The way the business views its processes is, first of all, much different from the way the BPM analysts do. Second of all, who said these analysts have the ability to design systems that will be functionally correct, reliable, scalable, and all that. So, why in the name of X are we giving them tools to automate that? Software architects don’t even have these kinds of tools, although they do have the requisite knowledge.

I’m with Steve. Bollocks!

Willam Brogden has an article up on SearchWebServices.com on How Web Services can use JavaSpaces. I don’t want all the Microsoft folks tuning out now that they’ve heard the “J” word, so let me just say that there are technologies out there for .NET too.

A “JavaSpace” is really just a space, which is, at the end of the day, a queryable distributed in-memory hashtable. Something many of us are already doing for caching. The reason you shouldn’t be doing this yourself is simple. While keeping a single hashtable in memory on a single computer and synchronizing it against changes to your database is simple, doing that in a highly available manner across multiple servers is not. Vendors providing solutions in this space include:

• Gigaspaces
• IBM ObjectGrid
• Alachisoft’s NCache
• GemStone
• Tangosol’s Coherence

But there are others as well. Bottom line: don’t develop one of your own. Do a proof of concept with your short list of vendors and go from there.

The article sums it up nicely like this:

Although JavaSpaces servers are not trivial to set up, they are much easier than any other type of grid computing server. Furthermore, the simplicity of the interface makes the learning curve easier. The greatest advantage of the JavaSpaces approach is the ease with which additional workers can be added to the grid.

It should be clear from the example that there is a lot of extra communication traffic in a JavaSpaces solution so the only reason to use JavaSpaces or any other form of grid computing in support of a Web service is a requirement for computing power or special resources that are not feasible to supply on the server directly.

I have this to add to it. Whereas most traditional systems keep the idea of message-based communication and data caching separate, spaces allow you to kill two birds with one stone. Even if you don’t go the whole Space-Based Architecture route, you’ll find that spaces will fit nicely in your distributed architecture toolkit – I know I did.

Roger Wolter, who is trying to evangelize the architectural aspects of data in SOA, has this to say:

After all, you don’t have to go too far down the SOA path before you realize that unless you build reliable, asynchronous, loosely-coupled services, your SOA architecture is going to have serious reliability problems and Service Broker brought reliable, transactional messaging to a whole new level of reliability and efficiency. What I found was a bunch of architects trying to figure out how to use WS-Transactions to build tightly coupled services to replace their tightly coupled objects.

So far, I’m thrilled that he is pushing this message in, and out of, Microsoft.

He then goes on to debunk the idea of Entity Services:

I next ran into a lot of people architecting SOA systems to provide a common services interface to a lot of diverse back-end systems. I’ve talked to people who had over 100 systems that handle customer data for example. If you build a perfect customer service to wrap all these systems with a common schema for the customer record you have a single view of the customer right? The first time your user tries to change the phone number for Acme Rockets Inc. and gets back 80 records which may or may not be for the same customer, the single view of the customer loses some of its appeal.

I’m loving it.

He then suggests a solution:

That’s how I got interested in Master Data Management. I really believe that accurate, unambiguous clean data is a prerequisite to an SOA project.

Hmm, from what I’ve seen of MDM, it puts a different spin on all the other things he said above.

Other than that, I’m totally with him. A deep understanding of data is necessary to get a good service decomposition. Without understanding the transactional nature of that data, you just might end up back tightly coupled and with a monolithic web service mess.

Whether or not you use Microsoft’s SQL Server Service Broker, Roger is definitely a guy worth listening to.

I know I bad-talk BizTalk a lot, but I do it somewhat tongue-in-cheek and to get a rise out of people. BizTalk is a tool, something of a swiss army knife, you know, those huge, honking ones that have everything but the kitchen sink in them. One of the main areas where BizTalk gets a bad rap is in performance and scalability. On the other hand, the number of systems that I come in to assist with performance issues, that don’t use BizTalk, is still quite large. A systematic approach is needed in all cases.

I ran into another BizTalk optimization story online that once again points out that disk IO is a good first place to look, as I wrote about in my Database performance optimization article.

In closing, you need to be aware of the full environment. For instance, in BizTalk, it’s not just about messages per second. Session contention (multiple parties hitting the same session) can just lock things up tighter than, well, a tight thing. Designing for these things “up front” can save you very costly rewrites later.

From a somewhat old email discussion I had with Benjamin Carlyle:

> A while ago I tried to explain the difference between SOA and REST is
> that SOA is based on “MEST” – MESsage Transfer. The main thing is the
> message – which is a statement of intent, and includes the relevant
> data for that intent. For instance, a message like
> ChangeCustomerAddressMessage is itself the intent; the data it
> carries, the customer ID and the new address completes the picture.

Ahh. This puts your name in context for me. I did some reading on MEST some time ago. I think that there are probably a few misunderstandings on both sides of the fence still about what the other is all about. I have recently been reading “Software Factories”, ISBN 0-471-20284-3. It takes the view of a service being a software component. Code usable in different software environments that has enough self-description to be accessed from multiple languages and runtimes. This sets up services as platform-independent objects.

I think I understand this evolution and the corresponding object patters that are encoded into WS standards. Essentially, SOA attempts to make objects more accessible. REST is quite a different beast. Its primary function is to allow communication to evolve.

> The main difference between this style and REST is that where REST is
> focusing on one resource, SOA/MEST style accept that fact that as a
> result in the change of address, the customer may now be in a “rich
> neighbourhood”, and his customer rep now has over X people in rich
> neighborhoods, so the customer rep’s status changes. This cascading of
> changes I believe is done in orchestration/choreography type code outside of the resource in REST.
> Such complex business logic is best implemented using OO models which
> span objects/entities/resources.

I think this is a common misunderstanding. Each resource demarcates a subset of an application’s state. Resources are likely to overlap. When one resource is updated, it is likely that other resources will also change. This is not well ennunciated by REST proponents who are far more focused on the other side of the exchange, that between client and server. In REST, your ChangeCustomerAddressMessage sent to object A would be a PUT of a customer address record over the top of the old one, at resource http://example.com/customer1.

I see REST as a tweak to the object model. Instead of having a base-class that defines methods and data to be transferred, REST decouples the definition of methods and data. The intention is that the set of methods, the set of data definitions, and the set of objects
(resources) can evolve independently of each other. This tweak does not change existing capabilities of the object model. Objects work pretty-much as before, but their interface to clients and the definition of their interface is modified.

The effect of the decoupling is that methods can be understood separately to data and to objects. For example, I can determine what a GET or PUT request is going to do to a resource regardless of the object the method is applied to. As an intermediatary I can operate as a caching proxy, or apply security policy appropriately. I see this like the jump to java beans. Some programmers were downright offended that they had to name their methods with “get” and “set” in order to get the architecture working, however the advantages are now so clear that the issue simply doesn’t come up anymore.

REST is about building a communicatons infrastructure that scales to a large number of participants who want different things from their architecture over time. At a technical level there are significant performance advantages to be gained by caching and other features, but at a social level it makes the definition of new interfaces simpler and allows them to change over time without disrupting the architecture as a whole. New methods can be introduced over time. New content types can be introduced over time. Old features can be supported simultaneously to new.

> It is this model of message/document based communication that isn’t
> very well supported by WSDL. Support for publish/subscribe semantics
> is also vary light in the WS world. This is the main reason why I see
> SOA as something outside of WS. I don’t see this as a “nod” to REST
> but a coherent architectural style that been employed long before the web.

Support for pub/sub is also pretty light in the REST world, something I have mind to do something about 🙂

> As for scalability, it’s very easy to scale the publishing of static
> html pages, it is an entire other thing to scale complex business
> logic that may have huge working sets of data. To tell you the truth,
> I’d be interested in seeing the design of such a system in terms of
> REST, specifically in how transactions are handled.

Transactions in REST are usually handled by demarcating all of the state that will be updated in a single resource, and PUTting a representation of that state to the resource. REST is usually applied at a chunky enough level that this makes sense. There is always the expectation in REST that objects or RDBMS or some other back-end technology does the work of making the updates.

There are other ways to handle transactions in REST, but they usually don’t come up. Again, it is a matter of scale and the level of abstraction. REST is a client-facing communications model. It isn’t necessarily useful for communication between services within the same tier. It may also be that the tier that presents a REST world view hides further tiers that use distributed object technologies.

> Anyway, that’s where I’m coming from. I think that the communications
> semantics of SOA – send a message, are just about as simple as it
> gets. It is, if anything, more minimalistic than REST.

I think that you are right about minimalism, at least after a fashion.
With WS-* or even corba, it is easy enough to make two programs work like one in an enclosed environment. REST is really about a larger-scale communications system where not all of the participants are part of the same organisation. It is about forging agreement on the larger scale, not an easy thing to do. However, I think that REST scales down better than SOA scales up. It is no harder to define special-purpose XML formats than it is to define a base-class, although defining all appropriate resources is probably starting to make life too difficult.

My organisation integrates disparate systems. It is about 50% of the work we do on a job. In this environment REST is a valuable tool in making things work with each other over long periods of time, and exposing the data of one system to another for viewing and update.

Orignal message:
> I’m quite interested in expanding my knowledge of SOA best practices.
> As I noted in the entry you refer to that my list is derived from a
> presentation by Sun Microsystems, the slides of which may be found href=”http://au.sun.com/events/developer/downloads/download.html?s_dLi
> nk=SOA _JBI_BPEL.pdf”>here. I often hear that SOA is an
> architectural style rather than a name for the WS-* stack, however it
> is hard to come to terms with what those principles are given the
> variety of views on the subject.
>
> It seems that many SOA proponents who are familiar with REST take a
> number of their particular constraints from REST itself. If those
> individuals are to be taken as the litmus test, SOA is generally a
> superset of REST that allows for less uniformity. In other words it
> looks like a version of REST that doesn’t scale up to such big network
> sizes. Vendors seem to see SOA differently, the message-bus focus I
> referred to in my entry.
>
> I would also like to note that the main focus of the title was on the
> principle difference that I percieve there to be between SOA and REST:
> the nature of the uniform interface. Even if you take your web
> services and try to construct a uniform interface, you do so as a
> base-class that encapsulates both verbs and content types. REST
> decouples these. I think the same is possible as document-oriented
> processing under web servcies, however this again seems to be a nod to
> REST rather than an alternative to REST practice.
>
> Do you have a specific list of constrains you yourself like to point
> to as the meaning of SOA?